Modeling carbonate-progradation geometry and sediment-accumulation rates--a comparison of "MARGIN" results with field data

byMark T Harris
Exxon Production Research Company

The geometries of carbonate-buildup margins vary widely but are commonly well exposed and clearly definable. Because carbonate buildups are, to a first approximation, relatively simple shapes, their geometries are readily amenable to quantitative computer modeling. This report considers the effects and constraints of different margin geometries and patterns of sediment accumulation on the progradation geometry of a carbonate buildup. This examination uses a simple sediment-accumulation model.

In most general terms, progradation of the margin requires sediment accumulation to fill in the edge of a basin, allowing a shallow-water platform to build over and bury the former basin and foreslope. In terms of sedimentation rates, progradation requires that foreslope accumulation exceed platform accumulation while escarpment formation implies the reverse. Thus, the large-scale pattern of buildup-progradation geometry reflects long-term patterns of sediment accumulation.

Models of prograding carbonate systems have typically focused on shallow-water, meter-scale, shallowing-upward packages (Read et al., 1986; Dunn et al., 1986; Spencer and Demicco, 1989). Some have attempted to model stratal patterns by modeling depositional processes (Lerche et al., 1987), or by using a sediment-budget model (Bice, 1988). The sediment-budget approach is adopted here because the consequences of different accumulation rates could be directly investigated.

A sediment-accumulation model

To examine margin geometries, "MARGIN" (a simple sediment-accumulation model of a carbonate buildup) was written in Pascal for an IBM PC. This model allows the comparison and evaluation of the significance of sediment-accumulation patterns and three-dimensional geometric factors in controlling the evolution of buildup geometries. The recognition of the more significant factors focused the consideration of actual field examples, here chosen from the Middle Triassic of the Dolomites.

The model uses a generalized buildup-margin profile consisting of a shallow-water flat-lying platform, a foreslope with a constant dip, and a deep-water flat-lying basin (fig. 1). The basic idea is similar to the model of Bice (1988): a series of iterations is run until a predetermined buildup thickness is reached. During each iteration, sediment is produced on the platform and is deposited on the platform up to sea level (filling any space created by subsidence), and any excess is added to the foreslope. However, two additional factors are added: 1) the three-dimensional geometry may be taken as a circular buildup or as a linear margin, and 2) a sediment-accumulation rate for basinal sedimentation. In addition, no attempt is made to duplicate the actual details of sediment transport or deposition but only the resulting sediment-accumulation patterns. Further details and the program code are presented in Harris (1988).

Figure 1--Basic modeling steps.

The three-dimensional shape (linear or circular), platform width (or radius), buildup relief, and foreslope angle define the initial buildup geometry. The subsidence history, sediment-production rate, and basinal sediment-accumulation rate control the addition of sediment during any simulation. A simulation terminates when the basin fills, the buildup drowns, or a preselected platform thickness is reached. The program calculates the principle geometric dimensions and the relative rates of sediment accumulation, and the results may be output as a scaled cross section of the buildup (fig. 2).

Figure 2--Buildup cross sections generated by the program "MARGIN" under conditions of constant subsidence Factors leo variatios in buildup geometry are indicated.

Changes in sediment-accumulation patterns during a simulation are the result of changes in the buildup geometry, subsidence rate, or sediment-production rate. Variations in the initial buildup geometry may also result in very different geometries at the end of a simulation.

Enhanced progradation results from various modeling conditions: 1) low buildup relief, 2) high foreslope angle, 3) slow subsidence, 4) high sediment production, 5) linear margin geometry, and/or 6) high rates of basin deposition (fig. 3). The first two factors reflect the smaller sediment volume required to fill in the edge of a low-relief, steep-sided basin. Factors 3 and 4 from above both increase the rate of foreslope deposition (and thus increase progradation). A linear margin progrades farther than a circular buildup because sediment production is proportional to the platform area , and a linear margin has a greater platform area for foreslope volume. A higher rate of basin deposition (factor 6) lessens the relief as the simulation proceeds.

During a modeling run, a decrease in subsidence rate will shift the maximum rate of deposition from the platform to the foreslope due to reduced sediment accommodation in the platform area. Increasing sediment productivity during a simulation has a similar effect. These effects may mimic changes in sediment-accumulation patterns during tectonic pulses or eustatic sea-level changes on a scale of a few million years or less.

On a long-term perspective (several million years, or many hundred of meters of thickness), the extent of progradation into basins depends upon the rate of basin filling. Starved basins ultimately become too deep for foreslope infilling in all simulations. Increasing foreslope deposition by factors of 2-3 will not maintain progradational geometries as the relief increases from tens to several hundred meters. This geometric relation suggests that the processes, thickness, and timing of basin sedimentation (which commonly may be siliciclastics or evaporites) is a controlling factor on the geometries of carbonate-margin progradation.

Comparison to ancient buildups

Despite the simplicity of the "MARGIN" model, the results provide insights for interpretation of ancient buildup geometries. Examples of two circular buildup geometries are briefly considered below.